Historical events linked to changes in Earth’s temperature

SHRINKING HOLE The ozone hole over Antarctica reached its largest extent in 2006 (purple and blue indicate depleted ozone) before starting to shrink. The international treaty intended to reduce the ozone hole has had a cooling effect on Earth’s climate, a study finds.

The Great Depression, World Wars I and II and an international treaty restricting ozone-depleting chemicals each had measurable effects on global temperatures, scientists report November 10 in Nature Geoscience. This finding represents one of the first times scientists have linked specific economic and political events to observed changes in how fast global temperatures are rising.

As greenhouse gas emissions soared over the last century, Earth’s temperature also rose, though not always at a constant rate. The planet’s average surface temperature grew steadily from 1910 to around 1940, cooled slightly until around 1970 and then shot up until 1998. In the last 15 years, the warming has slowed but not stopped, most climate researchers agree (SN: 10/5/13, p. 14). These shifts have both natural and human causes. But scientists have struggled to pinpoint the exact factors responsible for any particular change in Earth’s warming rate.

In the new work, climate change economist Francisco Estrada of the National Autonomous University of Mexico and colleagues used statistical methods they developed to test whether increases or decreases in greenhouse gas concentrations directly caused specific changes in the rate of global temperature rise. The researchers looked for times when these two rates changed abruptly and in the same direction. The scientists also removed from their analysis natural factors that affect Earth's temperature, such as a long-term oscillation of sea-surface temperature in the North Atlantic Ocean.

The scientists concluded that the world wars and the Great Depression, which collectively shuttered factories and reduced greenhouse gas emissions from 1914 to 1946, caused some of the cooling that began in the 1940s. Because carbon dioxide molecules stay in the atmosphere for decades, there was a time lag between when the emissions declined and when the planet’s temperature dropped.

The team also found that the 1989 Montreal Protocol, which phases out ozone-depleting chemicals, has helped slow global warming over the last 15 years. The treaty was intended to protect Earth’s ozone layer, but it has also had a cooling effect because most ozone depleters are chlorofluorocarbons, which, molecule for molecule, are among the most potent greenhouse gases.

Felix Pretis, a climate economist at the University of Oxford, applauds Estrada’s team for using a new statistical method to link changes in greenhouse gas concentrations with changes in warming rates. However, he wishes the team had also tried to estimate how much specific human activities, such as reducing chlorofluorocarbon emissions, influenced global temperatures. In an article accompanying Estrada’s in Nature Geoscience, Pretis and Oxford colleague Myles Allen test the team’s conclusions using a simulation that predicts temperature change resulting from various factors. They find that the world would be 0.1 degree Celsius warmer today had the Montreal Protocol not been enacted. Still, Pretis stresses that natural factors also contributed to the recent warming slowdown.